Flow cytometry was used to compare effects of caffeine or staurosporine on the radiation-induced G2-phase arrest and cyclin B1 expression in normal, human skin fibroblast (HSF) cells and HeLa cells following 6.0 Gy of gamma radiation. Caffeine and staurosporine were equally effective in attenuating both radiation induced increase in cyclin B1 expression and the prolongation of G2 in synchronous and asynchronous HeLa cell populations. Caffeine was also effective to a similar extent in irradiated HSF cells but staurosporine was ineffective in releasing the radiation-induced G2 block but did decrease cyclin B1 levels in those cell populations. These results suggest that staurosporine acts on different pathways of cell cycle controlling normal and transformed cells in response to radiation.